1. Field of the Invention
The present invention concerns a plane structure acoustic waves filter, that is in which acoustic waves spread inside a plane.
In particular, but not exclusively, it apples to wireless communication links like those used in mobile telephones or keys intended for the remote control for opening the doors of a motor vehicle, or even in the wireless exchange of data between delocalised computer or electric devices.
These wireless links use head filters, known as RF filters (Radio Frequency Filters) and FI intermediate frequency surface acoustic wave filters.
2. Description of the Prior Art
A surface acoustic wave filter is generally made up of a piezoelectric substrate which, under the effect of an electric field, generates acoustic waves which spread to the surface of the substrate, these waves being situated either inside the sagittal plane (Rayleigh waves) or orientated transversally (Bleustein-Gulayev waves) or quasi-transversally with respect to their propagation direction.
So as to generate these acoustic waves, the substrate includes a plane metal structure formed by a metal deposit laid on the substrate or imbedded in grooves formed on its surface, this plane metal structure including a plurality of fingers forming, for example, interdigitised combs, or forming an SPUDT type of transducer (Single Phase UniDirectional Transducer), a R-SPUDT type transducer (Resonant SPUDT) or acoustically or electrically coupled resonators. To obtain more information concerning these devices, reference can be made to the documents [4] to [8] mentioned in the list of references featured at the end of the present description and which are incorporated in the present description by way of reference.
The advantages of surface acoustic filters with respect to filters using another technique reside in their extremely small dimensions, low consumption (these are passive filters) and especially their cost of production.
However, the carrier frequencies used in mobile telephony are tending to increase from 900 MHz to 1800 MHz so as to reach 2200 MHz, indeed 3000 MHz and more. The size of the substrate, the gap between the fingers and in particular the size of the fingers of the metal structure thus need to be reduced accordingly so as to be able to process such high frequencies. Thus, the precision of lithography machines needed for carrying out metal etchings needs to change from 0.5 xcexcm to 0.35 xcexcm, indeed much less (0.25 to 0.18 xcexcm).
Furthermore, the presence of the least amount of dust or projection of a material damages the performances of the filter, all the more so when the dimensions of the substrate need to be increasingly reduced.
Thus, it is essential for the substrate to be protected by a box.
In addition, as the substrate is piezoelectric, it also needs to be protected from electromagnetic perturbations. As a result, the box needs to be rendered immune from interference.
Thus, there is a structure problem due to the cost of components since by becoming smaller and smaller, the acoustic chips are going to be much cheaper than the box intended to protect the chip from the environment.
The object of the present invention is to eliminate these drawbacks. To this effect, it offers a plane structure acoustic filter in which acoustic waves significantly spread inside a plane.
According to the invention, this filter is characterised in that it includes two solid bodies, one of these being piezoelectric, these bodies being linked to each other so as to have a plane interface between them, the acoustic waves being generated by the piezoelectric body with the aid of an electric field, the acoustic waves also being guided by the interface between, the two solid bodies and possessing an acoustic energy which decreases exponentially in the two bodies from the interface in a direction perpendicular to the interface.
By means of these arrangements, the acoustic waves spread, not to the surface of a substrate, but to the interface between two solid bodies. The energy of these waves decreases exponentially in the two solid bodies from the interface, although no energy goes outside the structure since the thickness of the solid bodies is greater than several ten times the wavelength of the acoustic waves. Accordingly, no box is required to protect the component (the filter) from its environment. Moreover, as the acoustic waves spread to the interface between two solid mediums, they can exhibit a speed of propagation greater than that of surface waves (for example 5000 m/s instead of 3000 m/s).
These interface waves, which are non-dispersive, can have two types of main polarisations: when the displacement vector of the atoms of the material is situated solely inside the sagittal plane (perpendicular to the interface plane), these are Stoneley waves, and when this displacement vector is situated solely along the direction perpendicular to the sagittal plane, these are Maerfel-Tournois transversal interface waves. A more detailed description of these waves can be seen in the documents [1], [2] and [3] mentioned at the end of the present description and which are integrated in the present description by way of reference.
Transversal or quasi-transversal waves are preferably used whose conditions of existence are less drastic than those of Stoneley waves and whose speed of propagation is generally greater.
Advantageously, the crystallographic axes of the materials constituting the two bodies are selected so that the transversal component of the displacement vector of the wave inside the plane of the interface is the only component of the displacement vector or is dominant and when the wave is coupled piezoelectrically.
One of the two bodies can to be made of a piezoelectric material or both. In the second case, the two piezoelectric materials can be identical. They can also be assembled so as to have the same crystalline orientations so as to form a single solid body. In this case, the plane metal structure forming the filter formed at the interface between the two bodies serves as a piezoelectric guide at the transversal interface wave (SH) or quasi-transversal interface wave (Q-SH).
When one of the materials is piezoelectric and the other not piezoelectric, the crystallographic axes of the piezoelectric material are preferably selected so that along the propagation direction the displacement vector of the acoustic waves is purely transversal-horizontal or has a dominant transversal-horizontal component and so that these waves are coupled piezoelectrically:
The crystallographic axes of the non-piezoelectric material can also be selected so that according to the selected direction of propagation, the displacement vector of the acoustic waves is purely transversal-horizontal or has a dominant transversal-horizontal component and so that these waves have a propagation speed equal to or greater than the speed of propagation of the transversal-horizontal or quasi-horizontal waves in the piezoelectric material.
According to one characteristic of the invention, the dielectric constant and the density of the non-piezoelectric material are respectively less than the dielectric constant and density of the piezoelectric material.